1. Field of the Invention
The present invention generally relates to a panel light device, and more particularly to a light guide of a back light module for a liquid crystal display.
2. Description of the Related Art
Referring to FIG. 1, it depicts the structure of a conventional liquid crystal display. Generally, the liquid crystal display device includes a liquid crystal panel 10 which has two substrates and a liquid crystal material sealed therebetween, a back light module 20 disposed, under the liquid crystal panel 10, and cases 11 and 12.
The back light module 20 is utilized to distribute the light from a light source uniformly over the surface of the liquid crystal panel 10. There are several kinds of back light modules 20 such as a direct back light type (or direct type) and an edge light type.
Referring to FIG. 2, which is a cross-sectional view along line 2—2 of FIG. 1, it depicts a back light module 21 of the direct type. The direct type back light module 21 includes a housing 70 which has a reflective sheet 60 disposed on the bottom surface of the housing 70 and, lamps 50, such as a fluorescence cathode tube, disposed at the bottom portion of the housing 70, a diffusing sheet 40 disposed on the top surface of the housing 70 and a prism sheet 30 disposed on the diffusing sheet 40. Since the lamps 50 are spacedly disposed inside the display area of the liquid crystal panel 10, the brightness distribution of the liquid crystal panel may be not uniform. Besides, the thin film transistors or the switching elements of the liquid crystal panel 10 may be damaged by the heat of the back light source.
Therefore, some distance or space between the lamp 50 and the diffusing sheet 40 is required. This distance or space is provided by means of supports 55 disposed in the housing 70. Typically, the distance between the center of the lamp 50 and the diffusing sheet 40 is required to be longer than a half distance between the adjacent lamps 50. Because of these structural requirements, it is very difficult to reduce the thickness of the direct type back light module 21. Furthermore, the lamp 50 also absorbs the light emitted from other lamps such that the optical efficiency is lowered.
Referring to FIG. 3, which is a cross-sectional view along line 2—2 of FIG. 1, it depicts a back light module 22 of the edge light type. The edge light type back light module 22 includes a light guide 80, a lamp 50 which is attached to at least one edge of the light guide 80, and a U-shaped reflector 61 which surrounds the lamp 50. An open portion of the reflector 61 is disposed at the edge of the light guide 80, a reflecting sheet 60 is disposed at the bottom of the light guide 80, a diffusing sheet 40 is disposed on the light guide 80 and a prism sheet 30 is disposed on the diffusing sheet 40. Because the lamp 50 is disposed at the edge of the light guide 80, the thickness of the LCD can be relatively decreased.
The light guide 80 includes a printed dot pattern or a V-shaped grooved pattern on one surface thereof and is typically made of PMMA by the process of molding. The diffusing sheet 40 is disposed on the light guide 80 and typically made of half-transparent polyethylene terephthalate (PET), polyethylene terephthalate, or polycarbonate for further evenly diffusing the light emitted from the light guide 80. The prism sheet 30 is disposed on the diffusing sheet 40 for gathering the light from the diffusing sheet 40 in the direction perpendicular thereto.
Since the liquid crystal panel 10 is illuminated by the light emitted from the light guide 80, the light guide 80 is required to uniformly or evenly illuminate over the liquid crystal panel 10. However, for the liquid crystal display of large dimensions in particular, the light guide 80 cannot provide even luminance. Furthermore, the manufacturing cost of the light guide 80 and the pattern thereon is relatively high.
For the back light module of the liquid crystal display in the prior art, there are many attempts made to increase the screen brightness, to provide uniform brightness, to lower power consumption and cost, as well as to decrease the dimension. Prior art attempts, for example, can be seen in U.S. Pat. No. 6,164,790 issued to Lee on Dec. 26, 2000, U.S. Pat. No. 5,477,422 issued to Hooker et al. on Dec. 19, 1995, and U.S. Pat. No. 5,485,354 issued to Ciupke et al. on Jan. 16, 1996. However, the back light modules in these patents and the other prior art still cannot meet the above-mentioned requirements.
Referring to FIG. 4a and FIG. 4b, they depict a back light module 23 having light sources of light emitting diodes in the prior art. The back light module 23 includes a light guide 80, a plurality of light emitting diodes 51 disposed beside the light guide 80, and reflectors 60, 62 respectively disposed on the bottom surface and on the side surface of the back light module 23. The back light module 23 cannot meet above-mentioned requirements, either, and particularly for the liquid crystal display of large dimensions, further cannot provide even light because the light emitting diode 51 is substantially a point light source instead of a linear light source, and located beside the back light module 23.
Accordingly, there exists a need for a back light module of a liquid crystal display capable of meeting the above-mentioned requirements.